This invention relates to hydrophones and more particularly to such hydrophones used in seismic streamers for seismic exploration under water.
Seismic streamers are usually made up from a plurality of sections each several hundred feet long, coupled together electrically and mechanically. Each section comprises a tube of flexible plastics material, such as polyurethane, several inches in diameter and about an eighth of an inch thick, within which a number of hydrophones are disposed at intervals of a foot or two along the length of the section. In between the hydrophones are other electrical components such as transformers. Bulkheads distributed along the length of the section, e.g. between each adjacent pair of hydrophones, keep the tube round so as not to press in on the hydrophones. Steel lines extend the length of the section to take tension imposed on the section, the lines extending inside the tube through passages in the bulkheads and alongside of the hydrophones. Electrical conductors extend from each hydrophone to one end of the section, and additional electrical conductors extend from the one end of the section to the other. The electrical conductors extend through passages in the bulkheads and alongside of the hydrophones. The streamer section is filled with oil to provide a desired overall specific gravity near to 1.0, and to keep salt water out of the tube and away from the electrical conductors and electrical components, and to transmit to the hydrophones pressure waves of acoustic and near acoustic frequency (hereinafter referred to simply as acoustic waves or frequency). The pressure waves in the water in which the tube is immersed pass through the flexible tube and through the oil in the tube to the hydrophones. When the section is not in use it is wound on a reel that is several yards in diameter.
It will be apparent that a hydrophone employed in a streamer section must have enough strength and toughness to withstand a certain amount of abuse during reeling and unreeling. In addition, the hydrophone must have enough rigidity to minimize mechanical strains, keeping them small in comparison to acoustic deformations of the pressure sensitive diaphragms of the hydrophone. Furthermore, the diaphragms must be shielded from the tension lines and electrical conductors which pass alongside of the hydrophones, since these linear elements may exert considerable lateral force when pulled taught during use. It is also desirable to protect the electrical parts of the hydrophone, i.e. the piezoelectric elements mounted on the diaphragms, from mechanical destruction due to contact with the tension lines and electrical conductors. Finally, the hydrophone's gas (air) chamber, must be sealed liquid tight to prevent entrance of oil or other liquid into the chamber, for such liquid would fill up the chamber and render the hydrophone insensitive to acoustic waves. On the other hand, means must be provided to limit the inward travel of the diaphragms under excessive pressure, e.g. due to the streamer being subjected to excessive hydrostatic pressure.
In response to the foregoing requirements it has heretofore been disclosed that a hydrophone may be constructed as described in U.S. Pat. No. 3,970,878--Berglund the disclosure of which is incorporated herein by reference. According to that patent a hydrophone comprises a pill box shaped chamber housing peizoelectric wafers on the inside of the top and bottom of the case. In one embodiment the case includes a glass ring to which are bonded diaphragms made of beryllium-copper, phosphorbronze, stainless steel or glass. According to a commercially adopted embodiment of the construction shown in the Berglund patent, the pill box shaped chamber is formed by two brass cups placed rim to rim and soldered together. Piezoelectric wafers are conductively secured to the insides of the flexible diaphragms forming the bottoms of the cups. Flanges extending radially from the rims of the cups are received within elastomeric channels provided inside a protective housing. Within the chamber formed by the cups is a plastics material spacer which limits inward displacement of the diaphragms and the wafers secured thereto. The brass cups themselves serve as one pole of the hydrophone, being connected internally to the outer side of each wafer. The inner sides of the wafers are tied together and a conductor connected thereto is brought out through a glass bushing in a hole in one of the cups. The chamber formed by the cup is thus composed entirely of corrosion resistant metal and ceramic material so that it is impervious to destruction by oil or any salt water which may leak into the tube of the streamer section or by any corrosive gas associated therewith, whereby the chamber remains fluid tight.
Although the foregoing construction has been highly successful commercially, the thought has occurred that its construction would be improved if the plastics material spacer could be eliminated. The persent invention accomplishes that result and in addition eliminates the outer protective housing and considerably reduces the size and cost of the hydrophone. At the same time all of the advantages of the prior construction are retained.